This invention relates to a magnetic disc apparatus, and more particularly, to a self-compensating device for a magnetic disc apparatus which can automatically compensate for a change of the characteristic of an actuator for positioning a magnetoelectric transducer.
The magnetic disc apparatus employs at least one rotatable magnetic disc as a recording medium. The recording surface of the disc has a number of data tracks concentrically disposed thereon. Data is serially recorded along the respective tracks. Provision of a read/write transducer for each track is effective for reduction of the access time but uneconomical. Therefore, the magnetic disc apparatus is usually provided with at most each or several transducers for one recording surface, each transducer being used for writing data into or reading out data from a plurality of the tracks.
A magnetic disc apparatus of the above type is provided with a transducer positioning mechanism to reciprocally move the transducer across the data tracks for positioning the transducer above the desired or target track. The transducer positioning mechanism is generally provided with a reciprocally movable carriage, and an actuator for actuating the carriage.
When the velocity of the transducer is too low, the access time of course increases. Conversely, when the velocity is too high, the access time also increases. A velocity which is too high makes it difficult to stop the transducer within the distance from the transducer to the target track, so that the transducer often moves beyond the target track. Therefore, in order to accurately position the transducer at and above the target track in the shortest time, it is necessary to move the transducer at a velocity selected corresponding to the distance from the transducer to the target track. For this reason, most of the magnetic disc apparatus currently used are each provided with a velocity servo system to keep the transducer velocity at a given target velocity.
The velocity servo system is usually comprised of a velocity command signal generator for determining a proper velocity (target velocity) in accordance with the distance from the transducer to the target track, a velocity detector for detecting an actual velocity (real velocity) of the transducer, a velocity error detector for detecting a difference between the target velocity and the real velocity, and an actuator for accelerating or decelerating the transducer in response to the output signal of the velocity error detector.
In case an actuator of the velocity servo system has an unexpected characteristic such as a current-acceleration characteristic, the access time increases or the operation of the velocity servo system becomes unstable. In order to eliminate such defects, an apparatus to electrically compensate for a change of the characteristic of the actuator and thus for coping with a change of the characteristic of the actuator has been used in the velocity servo system.
Conventionally, a compensator of the above-described type is manually adjusted, and adjustment is complicated and troublesome. This leads to inefficiency in the manufacturing of the magnetic disc apparatus. Further, the conventional compensating method can not easily compensate for a change of the characteristic of the actuator arising from some causes occurring after the system is in operation such as aging of the actuator or replacement of the actuator.